mysqli::select_db函數的基本語法是什麼？詳細講解使用方法

gitbox 2025-08-11

<?php // 这部分代码与文章内容无关，仅作为示例 echo "欢迎阅读本篇PHP教程。"; ?> <hr> <h1>mysqli::select_db函数的基本语法是什么？详细讲解使用方法</h1> 在PHP中，使用MySQLi扩展进行数据库操作时，切换数据库是常见的需求。<code>mysqli::select_db

：要切換到的數據庫名稱。
返回值為TRUE表示切換成功， FALSE表示失敗。

使用說明

當你使用new mysqli(...)連接數據庫時，通常會指定一個默認數據庫。但在某些情況下，你可能需要在同一個連接中切換不同的數據庫，這時就可以調用select_db 。

示例代碼

&lt;?php
</span><span><span class="hljs-comment">// 創建MySQLi連接，默認數據庫為db1</span></span><span>
</span><span><span class="hljs-variable">$mysqli</span></span><span> = </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-title function_ invoke__">mysqli</span></span><span>(</span><span><span class="hljs-string">"localhost"</span></span><span>, </span><span><span class="hljs-string">"root"</span></span><span>, </span><span><span class="hljs-string">"password"</span></span><span>, </span><span><span class="hljs-string">"db1"</span></span><span>);

</span><span><span class="hljs-comment">// 检查連接是否成功</span></span><span>
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-variable">$mysqli</span></span><span>->connect_error) {
    </span><span><span class="hljs-keyword">die</span></span><span>(</span><span><span class="hljs-string">"連接失败: "</span></span><span> . </span><span><span class="hljs-variable">$mysqli</span></span><span>->connect_error);
}

</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"當前數據庫為: db1\n"</span></span><span>;

</span><span><span class="hljs-comment">// 切換到db2資料庫</span></span><span>
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-variable">$mysqli</span></span><span>-></span><span><span class="hljs-title function_ invoke__">select_db</span></span><span>(</span><span><span class="hljs-string">"db2"</span></span><span>)) {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"成功切換到資料庫 db2\n"</span></span><span>;
} </span><span><span class="hljs-keyword">else</span></span><span> {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"切换資料庫失败: "</span></span><span> . </span><span><span class="hljs-variable">$mysqli</span></span><span>->error;
}

</span><span><span class="hljs-comment">// 後續可以在db2上執行查詢</span></span><span>
</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-variable">$mysqli</span></span><span>-></span><span><span class="hljs-title function_ invoke__">query</span></span><span>(</span><span><span class="hljs-string">"SELECT * FROM table_in_db2"</span></span><span>);
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-variable">$result</span></span><span>) {
    </span><span><span class="hljs-keyword">while</span></span><span> (</span><span><span class="hljs-variable">$row</span></span><span> = </span><span><span class="hljs-variable">$result</span></span><span>-></span><span><span class="hljs-title function_ invoke__">fetch_assoc</span></span><span>()) {
        </span><span><span class="hljs-title function_ invoke__">print_r</span></span><span>(</span><span><span class="hljs-variable">$row</span></span><span>);
    }
} </span><span><span class="hljs-keyword">else</span></span><span> {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"查詢失敗: "</span></span><span> . </span><span><span class="hljs-variable">$mysqli</span></span><span>->error;
}

</span><span><span class="hljs-variable">$mysqli</span></span><span>-></span><span><span class="hljs-title function_ invoke__">close</span></span><span>();
?&gt;

注意事項

調用select_db前，連接必須已成功建立。
切換數據庫不會關閉當前連接，而是更改連接使用的數據庫上下文。
如果指定的數據庫不存在，函數會返回FALSE ，並通過$mysqli->error獲取錯誤信息。

總結

mysqli::select_db是MySQLi面向對象方式中切換數據庫的函數，適用於同一連接需要操作多個數據庫的場景。掌握它可以使你的數據庫操作更加靈活。

相關標籤:
mysqli

mysqli::select_db